With external drives, it's always possible to get one that has a signature collision with one you already have. If this happens, when you plug it in, you won't see it come online and, if you pull up the Disk Management console, you'll see it is marked Offline with a help bubble that indicates, "The disk is offline because it has a signature collision with another disk that is online." [more]
Of course, this typically happens when a disk is cloned and the disk signature is duplicated.
You can right-click on the drive and select "Online" to have Windows generate a new random disk signature and assign it to the disk by writing it to the MBR.
This is all you need to do unless this was a bootable disk and you want it to remain bootable. When this new signature is written to the MBR, the Boot Configuration Database (BCD) entries become orphaned, linked with the previous disk signature rather than the new one. If orphaned BCD entries are causing problems, there are some options (such as giving a disk its original signature back) documented by Mark Russinovich at the following URL: http://blogs.technet.com/b/markrussinovich/archive/2011/11/08/3463572.aspx